As a method of manufacturing seamless steel tubes under hot working conditions, the Mannesmann tube-making process is widely employed. In this tube-making process, a round billet heated to a high temperature is fed as a material to be rolled into a piercing mill (a so-called "piercer"), which pierces the axial center portion of the round billet to obtain a hollow shell. The thus-obtained hollow shell is fed, directly or as needed after undergoing an expansion or wall-thinning process in an elongator having the same structure as that of the piercing mill, into a subsequent elongating mill such as a plug mill, a mandrel mill, or the like so as to be elongated. Subsequently, the thus-elongated tube undergoes a finishing process provided by a stretch reducer for shape correction, a reeler for polishing, and a sizer for sizing, thereby becoming a seamless steel tube product.
FIG. 1 shows the material being pierced and an arrangement of piercing rolls and disk rolls in a piercing mill. Piercing rolls 2, 2 are axisymmetrically arranged such that each of the piercing rolls 2, 2 forms a predetermined cross angle and inclination angle with respect to the pass line X--X along which a round billet 1 serving as the material is rolled. Disk rolls 3 are arranged in proximity to and in a manner perpendicularly intersecting with the piercing rolls 2 such that the disk rolls 3 are opposed to one another with the round billet held in-between, and rotate independently of each other about respective shafts 4. In the piercing mill with the piercing rolls 2, 2 arranged as above, when the round billet 1 is fed along the pass line X--X in the direction indicated by the illustrated white arrow, the round billet 1 is caught between the piercing rolls 2 and is transferred along the pass line X--X while being rotated, during which the round billet 1 is pierced at its axial center portion by an unillustrated plug so as to become a hollow shell. During this piercing process, the disk rolls 3 follow the movement of the round billet 1 and rotate in the direction of the pass line X--X so as to prevent the round billet 1 from deflecting, to thereby establish smooth piercing.
In general, a piercing mill employing such a roll arrangement as shown in FIG. 1, in which a pair of piercing rolls are opposingly arranged in a vertical direction with a pass line running in-between, and a pair of disk rolls are opposingly arranged in a horizontal direction, is usually called a "vertical piercing mill." In recent years, vertical piercing mills have widely been used in a seamless steel tube manufacturing process.
As described above, since piercing mill forces the high-temperature material between piercing rolls, the working surfaces of the piercing rolls are degraded with rolling time. Also, since disk rolls are in contact with the material during rolling, their surfaces unavoidably wear. Accordingly, the piercing rolls and the disk rolls must be replaced periodically. Particularly, exchanging disk rolls involves the steps of opening a mill housing and lifting the disk rolls up one after another for exchange through use of an overhead crane. Since exchanging a pair of disk rolls through use of an overhead crane takes a considerable amount of time, a reduction in the working ratio of the piercing mill is unavoidable.
Various improvements have been proposed for solving the above-described problem involved in disk roll exchange. For example, Japanese Patent Application Publication (kokoku) No. 63-64248 discloses the structure of a piercing mill which allows disk rolls to be replaced without use of an overhead crane. However, this proposed structure is only applicable to the case where a disk roll is supported in a cantilever manner on the driving shaft side. This cantilever-like support of a disk roll has the problem that the disk roll is not positioned in a sufficiently rigid manner during operation. That is, since the cantilever-like support of a disk roll involves a reduction in supporting rigidity, the disk roll may deflect from the material during piercing, resulting in the marking of scratches on the material surface.
By contrast, when a disk roll is supported at both ends of its shaft in order to secure a sufficient supporting rigidity for the disk roll, the structure of a supporting apparatus for a disk roll becomes complex. Thus, a disk roll exchanging apparatus applicable to such a complex supporting apparatus becomes large-scaled, resulting in a potential significant reduction in workability of disk roll exchange. This reduction in workability of disk roll exchange affects not only the working ratio of a piercing mill but also an overall efficiency of manufacture of seamless steel tubes, particularly when a continuous Mannesmann tube-making process is carried out for high efficiency production of seamless steel tubes as has been practiced in recent years.